This invention relates to an ignition timing control apparatus for an internal combustion engine, and in particular to an ignition timing control apparatus for an internal combustion engine which is operable either with a low-octane rating fuel or a high-octane rating fuel, or with a mixture thereof.
It is well known in the art that the octane rating of fuel gasoline is greatly interrelated with a knock-resistibility in an internal combustion engine. Namely, as the octane rating of gasoline becomes higher, the knocking of an engine correspondingly decreases.
FIG. 1 shows an ignition timing vs output shaft torque characteristic in an internal combustion engine in the case where a commercially available low-octane gasoline (regular gasoline) and a commercially available high octane gasoline (premium gasoline) of which the octane rating is higher than that of low-octane gasoline. In the figure, point A indicates a knock limit for low-octane gasoline, point B indicates a knock limit for high-octane gasoline, and point C indicates a Minimum advance for Best Torque (MBT), in which knock arises when the angle of the ignition timing is advanced beyond these points.
According to FIG. 1, it is possible to advance the angle of the ignition timing up to a point B when high-octane gasoline is used, where the output shaft torque will be higher as compared with the use of low-octane gasoline.
FIGS. 2 and 3 show ignition timing characteristics of points A, B, and C in FIG. 1 respectively represented as a function of a revolutional speed of an internal combustion engine (FIG. 2) and a function of a load of the engine (FIG. 3).
Thus, in an engine associated with such a characteristic, in the case where regular and premium gasolines are alternatively used and the ignition timing of the engine is changed over to have an optimum value according to the kind of gasoline in use, or in the case where a mixture of both gasolines is used and the ignition timing is advanced corresponding to the mixing ratio of regular and premium gasolines, the output of the engine can be enhanced.
However, in a conventional control apparatus for an internal combustion engine, the reference ignition timing characteristic is preset only for a predetermined octane rating gasoline, e.g. the low-octane rating regular gasoline. Therefore when the gasoline of the engine is changed over from the low octane rating gasoline to the high octane rating gasoline or when both of the gasolines are mixed in use, an increase in the output of the engine can not be expected without any change or modification thereof, so that the presetting of the reference ignition timing must be repeated towards an advance angle, by some method.
Particularly, such a repeated presetting of the reference ignition timing is very hard when using mixed gasoline because the knock limit exists between curves A and B depending on the mixture ratio and so the possible advance angle limit is variable.
Moreover, upon the presetting, an advance angle characteristic different from that of the regular gasoline is required to be set as the ignition timing because of the difference in the advance angle characteristic between the knock limits A, B and the MBT point C as shown in FIGS. 2 and 3.
It has been also considered that by means of a knock control technique, which will be hereinafter described in the embodiments of this invention, for detecting engine knocks and controlling the ignition timing to the knock occurrence limit, the reference ignition timing characteristic be preset to point B in FIG. 1 for premium gasoline and that when using the regular gasoline, the ignition timing be retarded to point A in FIG. 1 by means of the above knock control technique. In this case, however, because of the broad ignition timing interval for a knock control between points A and B in FIG. 1, when knocks arise during an acceleration operation etc., of the engine, intensive knocks disadvantageously arise during the retarding shift of the ignition timing from point B to point A in FIG. 1.
Even if the reference ignition timing has been again preset at the knock limit A or B, since the knock limits A and B in FIG. 1 vary with environmental conditions during the operation of the engine such as temperature, humidity, etc, there is a possibility that knocks will arise due to such variations in environmental conditions.
Furthermore, depending on the operating region of the engine, there is a case where an accurate determining operation of an octane rating can not be done.
U.S. Pat. No. 3,822,583, titled "Method for Determining Octane Rating of Fuels under Road Conditions" discloses measuring the octane rating of a fuel used for an engine by the detection of knocks.
Japanese Patent Application Laid-open No. 58-57072, published on Apr. 5, 1983, titled "Method for Controlling Ignition Timing of Electronic Controlled Engine" discloses a change-over the ignition timing of the engine depending on the octane rating of a fuel.